Answer by Rohat Kılıç for Can I supply 3.3V to a 4V stepper motor?

Can I supply the motor with a 3.3V voltage for simplicity

Well, you can but you'll lose torque and maximum speed.

... or do I need two separate buck converters: a 4V for the motor and a 3.3V for the STM32?

If 3.3V is needed for only the MCU then you can use a single DC-DC converter for 4V, and use an LDO (low dropout) regulator to get 3.3V from 4V. There are many options available with a dropout voltage of as low as 400 mV.

^{simulate this circuit– Schematic created using CircuitLab}